1. Field of the Invention
This invention relates to the oil exploration industry and particularly to the area of neutron well logging.
2. Prior Art
Nuclear techniques have been extensively applied to the problems of the oil exploration industry, particularly in the area of well logging. Nuclear particle detectors are used to differentiate between strata containing oil, natural gas, water and the type of strata containing these fluids. These investigations estimate the extent of the oil-bearing strata and the amount of oil that these strata contain.
The scientific basis for this survey technique is based on the difference in the neutron reactions of the various materials. The varying reactions are monitored by the detection of gamma rays or neutrons returning from the strata. A probe containing a neutron source is drawn through the bore hole and a detector measures the energy and intensity of the emitted incident particles. Oil, gas, water and various geological formations possess a distinctive gamma ray signature that permit identification of the chemical constituents of the strata.
Two types of neutron sources are now used in the downhole probe. One type employs radioactive sources, such as americium/beryllium or californium, that continuously emit neutrons. The other employs a pulsed 14-MeV neutron generator. Each of these methods has serious drawbacks. The radioactive source requires special licenses in every country in which the source will be used, and must have massive shielding and complicated transportation arrangements. Most importantly, radioactive sources are occasionally lost downhole, which can potentially cause the loss of the hole. The radioactive sources, however, possess the advantage of a well known neutron emission rate.
The pulsed generators are safer to use and allow a wider variety of measurements downhole. The disadvantage of pulsed sources is that they may not emit neutrons at a constant rate.
A high efficiency detector has been proposed for the measurement of short pulses of d+T neutrons. It is based on the activation of sodium in a sodium iodide scintillation detector via the .sup.23 Na (n,.alpha.) .sup.20 F reaction and the immediately postpulse measurement of the induced beta activity which has an 11 second half life. The detector is insensitive to neutrons with energies below about 7 MeV, and may be calibrated directly against an absolute technique using a "steady current" d+T neutron generator. It is noted that the threshold required is low and encompasses the oxygen background, thereby greatly reducing the usefulness of the method disclosed. V. E. Lewis and T. B. Ryves Nuclear Instruments and Methods in Physics Research A 257 (1987) 462-466.